This research primarily concerns the role of the cytoskeleton in the acute responses of cultured adrenocortical tumor cells to ACTH. Information obtained by various forms of electron microscopy and 16mm cinemicrography is correlated with pertinent biochemical data. We have recorded general ultrastructural changes in cultures stimulated for 15 minutes to 2 hours with ACTH. Although these cells retain an essentially dedifferentiated ultrastructure, their characteristic "rounding up" is accompanied by an increase in the number of filament-containing surface features, a hypertrophy of the Golgi complex, intensified pinocytosis and rearrangement of lysosome-like organelles. As seen by stereoscopic high voltage electron microscopy, microtubule polymerization occurs and a loss of stress fibers is associated with a more random arrangement of the microtrabeculae. These results have provided the background for further investigations of the shape change and increased steroid output of acutely stimulated cells. In this work, we are employing inhibitors of microfilaments and microtubules. These agents may help to establish how the cytoskeletal elements interact with each other or with certain other organelles implicated in steroid biosynthesis and secretion. Cells incubated with ACTH for longer time periods (several days) exhibit a more specialized ultrastructure. Possible trophic effects of the hormone on cytoskeletal arrangements and functions are being explored.